Allow black slider to take negative values, allows shadow lifting.

rtengine/curves.h

@@ -27,6 +27,8 @@
 
 #define CURVES_MIN_POLY_POINTS  1000
 
+#define SQR(x) ((x)*(x))
+
 namespace rtengine {
 
 class CurveFactory {
@@ -82,17 +84,35 @@ class CurveFactory {
         if (x<x1) return x*m;
         return 1.0 - hr + hr*baseu((x-x1)/hr, m, 0.3*hr);
     }
+	static inline double clower2 (double x, double m, double sr) {
+		float x1 = sr/1.5 + 0.00001;
+		float y1 = 1-(1-x1)*m;
+		if (x>x1 || sr<0.001) 
+			return 1-(1-x)*m;
+		else
+			return y1+m*(x-x1)-(1-m)*SQR(SQR(1-x/x1));
+	}
     // tone curve base. a: slope (from exp.comp.), b: black, D: max. x value (can be>1), hr,sr: highlight,shadow recovery
     static inline double basecurve (double x, double a, double b, double D, double hr, double sr) { 
-        double m = a>1 ? b+0.25*(1-b)/a : b+(1-b)/4;
-        double y = a>1 ? 0.25 : 0.25*a;
-		double slope = a/(1-b);
-        if (x<=m)
-            return b==0 ? x*a : clower (x/m, slope*m/y, sr) * y;
-        else if (a>1)
-            return y+(1.0-y)*cupper2((x-m)/(1-m), slope*(1-m)/(1.0-y), hr);
-        else
-            return y+(x-m)*slope;
+        if (b<0) {
+			double m = 0.5;
+			double slope = 1+b;
+			double y = -b+m*slope;
+			if (x>m) 
+				return y + (x - m)*slope;
+			else 
+				return y*clower2(x/m, slope*m/y, 2.0-sr);
+		} else {
+			double m = a>1 ? b+0.25*(1-b)/a : b+(1-b)/4;
+			double y = a>1 ? 0.25 : 0.25*a;
+			double slope = a/(1-b);
+			if (x<=m)
+				return b==0 ? x*a : clower (x/m, slope*m/y, sr) * y;
+			else if (a>1)
+				return y+(1.0-y)*cupper2((x-m)/(1-m), slope*(1-m)/(1.0-y), hr);
+			else
+				return y+(x-m)*slope;
+		}
     }
     // brightness curve at point x, only positive amount it supported
     static inline double brightnessbase (double x, double amount) {

rtengine/improcfun.cc

@@ -346,9 +346,9 @@ void ImProcFunctions::rgbProc (Image16* working, LabImage* lab, int* hltonecurve
 			b *= tonefactor;
 			
 			//brightness/contrast and user tone curve
-			r = tonecurve[r];
-			g = tonecurve[g];
-			b = tonecurve[b];
+			r = tonecurve[CLIP(r)];
+			g = tonecurve[CLIP(g)];
+			b = tonecurve[CLIP(b)];
 
 			if (abs(sat)>0.5) {
 				float h, s, v;

rtgui/tonecurve.cc

@@ -55,7 +55,7 @@ ToneCurve::ToneCurve () : ToolPanel(), expAdd(false), blackAdd(false), brAdd(fal
   pack_start (*hlcompr);
 
 //----------- Black Level ----------------------------------
-  black = Gtk::manage (new Adjuster (M("TP_EXPOSURE_BLACKLEVEL"), 0, 32768, 1, 0));
+  black = Gtk::manage (new Adjuster (M("TP_EXPOSURE_BLACKLEVEL"), -16384, 32768, 1, 0));
   pack_start (*black);
   shcompr = Gtk::manage (new Adjuster (M("TP_EXPOSURE_COMPRSHADOWS"), 0, 100, 1, 50));
   pack_start (*shcompr);